[Zancarius]

strlen and strpos are two fairly commonly used functions that come to mind.

[desas]

Use mb_strlen and mb_strpos instead where the string may contain multibyte characters.

[Zancarius]

Or patchwork/utf8 [1] since it handles fallback in the event mb_* isn't installed and can utilize several other libs.

[1] https://github.com/nicolas-grekas/Patchwork-UTF8